Carbohydrates, acidic derivatives stability

Methods for the regioselective radical-mediated bromination of carbohydrates have been reviewed by Somsak and Perrier [95]. Photobromination of hexuronic acid derivatives with non-activating anomeric substituents leads preferentially to formation of the C-5 brominated product via the stabilized captodative radical at C-5. 

Chiral ferroceneboronic acid derivatives have been synthesized by Ori and tested for chiral electrochemical detection of monosaccharides. The best discrimination was observed for L-sorbitol and L-iditol at pH 7.0 in 0.1 mol dm phosphate buffer solution. Moore and Wayner have explored the redox switching of carbohydrate binding with commercial ferrocene boronic acid. From their detailed investigations, they have determined that binding constants of saccharides with the ferrocenium form are about 2 orders of magnitude greater than those for the ferrocene form. The increased stability is ascribed to the lower pKe of the ferrocenium (5.8) than ferrocene (10.8) boronic acid. 

Another approach for the in situ generation of aldehydes in the presence of stabilized phosphonium ylides is the oxidative cleavage of glycols using NaI04 on silica gel (Scheme 19) [99]. The simultaneous, one-pot oxidative cleavageAVittig reaction of carbohydrates and amino acid derivatives affords a number of synthetically useful alkenes with high E selectivity. 

Nitroanilides, l-acyl-7-nitroindolines, and A -acyl-1,2,3,4-tetrahydro-8-nitroquinolines are readily photolyzed to yield the free carboxylic acids (80-100%) and photoproducts. These nitro derivatives (see Fig. 2) possess higher stability to acid hydrolysis and a lower stability to basic hydrolysis than non-nitrated anilines. Nevertheless, they are unlikely to prove of service to the carbohydrate chemist, unless they can be synthesized from carboxylic acids under milder conditions than those devised to date. 

Vitamin C, also known as L-ascorbic acid, clearly appears to be of carbohydrate nature. Its most obvious functional group is the lactone ring system, and, although termed ascorbic acid, it is certainly not a carboxylic acid. Nevertheless, it shows acidic properties, since it is an enol, in fact an enediol. It is easy to predict which enol hydroxyl group is going to ionize more readily. It must be the one P to the carbonyl, ionization of which produces a conjugate base that is nicely resonance stabilized (see Section 4.3.5). Indeed, note that these resonance forms correspond to those of an enolate anion derived from a 1,3-dicarbonyl compound (see Section 10.1). Ionization of the a-hydroxyl provides less favourable resonance, and the remaining hydroxyls are typical non-acidic alcohols (see Section 4.3.3). Thus, the of vitamin C is 4.0, and is comparable to that of a carboxylic acid. 

The major limitation of GC is the requirement for heat stability and volatility of the sample. Obviously, compounds that decompose at elevated temperatures (below 250°C) cannot normally be subjected to GC analysis. Many compounds of biochemical interest are not volatile in the useful temperature range of GC (up to about 200-250°C). Such compounds can often be converted to volatile derivatives. Hydroxyl groups in alcohols, carbohydrates, and sterols are converted to derivatives by trimethylsilylation or acetylation. Amino groups can also be converted to volatile derivatives by acetylation and silylation. Fatty acids are transformed to methyl esters for GC analysis, as described in Experiment 6. 

The carbonate salts investigated by Siegfried and Howwjanz were decomposed instantly by dilute acetic acid, whereas the moderate stability of the 0-(sodium thiolthiocarbonyl) derivatives toward this reagent has been a valuable aid to their purification. In mineral acids, however, the 0-(metal thiolthiocarbonyl) compounds are also rapidly attacked, with regeneration of the carbohydrate, a property utilized in the cellulose Viscose process. The 0-(sodium thiocarbonyl) derivative of cellulose is less resistant to acid hydrolysis than its thiolthiocarbonyl analog. 

Derivatization of non-volatile polar or thermally sensitive compounds to enhance their volatility and stability prior to chromatography is a well-established technique. Compounds containing hydroxyl, carboxyl and amino functional groups can be readily reacted with appropriate reagents to convert these polar groups into much less polar methyl, trimethylsilyl or trifliioroacetyl derivatives of greater volatility. Fatty acids. Carbohydrates. 

Since the complex in which this ligand is coordinated only by carboxylate-0 and amide-N atoms would not be optically active, CD spectroscopy was used to confirm the coordination of the alcoholic OH group on C(2) of the sugar moiety to copper(Il), resulting in the formation of a second chelate ring, beside that between the carboxylate-0 and amide-N donor atoms. Similar extra stabilization was observed in complexes of carbohydrate derivatives of thiazolidine-4-carboxylic acid.2.3 This effect was shown to depend on the conformation of the sugar moiety. 

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